1. Field of the Invention
The present invention relates to a projection type display device wherein optical images formed on a light valve are irradiated by light and are projected on a screen via a projection lens.
2. Disclosure of the Prior Art
As a large size display device for visual images, there have been developed projection type display devices wherein optical images are formed on a small light valve based on visual image signals and irradiated with light and wherein the thus irradiated optical images are enlarged by means of a projection lens to be projected and displayed on a rear projection image display screen. Conventionally, CRTs (cathode-ray tubes) have been used as the light valve, but more recently liquid crystal display panels on which the pixels are arranged in matrix are more frequently used.
Two-sheet screens comprising a combination of a Fresnel lens and a lenticular lens are generally used as the rear projection screen (e.g. U.S. Pat. No. 4,432,010). FIG. 1 is a schematic perspective view showing a rear projection image display device using said two-sheet screen. In FIG. 1, a light valve 2 comprising a liquid crystal display panel is irradiated with the light from an irradiation system 1. Optical images formed on the liquid crystal display panel 2 are enlarged by a projection lens 3, and the enlarged images are formed on a two-sheet screen 6 comprising a Fresnel lens 4 and a lenticular lens 5. The Fresnel lens 4 has functions to direct the incident light toward the viewer. The lenticular lens 5 disperses the light emerging from the Fresnel lens 4 in the horizontal and vertical directions at any desired angles and in any desired ratios.
The pixels in the liquid crystal display panel may be arranged either in stripes or in a delta matrix. FIG. 2 is a schematic plan view to show one embodiment of a liquid crystal display panel wherein each pixel contains three primary colors. In the liquid crystal display panel system with a delta matrix shown in FIG. 2, a composite color of red (R), green (G) and blue (B) is displayed in one LCD pixel. In the liquid crystal display panel with a delta matrix as shown in FIG. 2, one liquid crystal pixel is provided for each of the colors red (R), green (G) and blue (B). As the three colors are combined on the incident side of the projection lens, images are displayed in full color. The liquid crystal display panel system with a delta matrix having this construction is called a three-panel system and is described, for example, in "Optical system of the HDTV rear projector using LCD panels, Kato et al., ITEJ Technical Report Vol. 16, No. 13, pp. 65-70 (1992).
FIG. 3 is a schematic plan view showing an embodiment of a liquid crystal display panel with delta matrix wherein each pixel contains only one color pixel out of the three primary colors. In this type of liquid crystal display panel, one LCD pixel displays one color in red (R), green (G) or blue (B). The liquid crystal display panel with a delta matrix having this construction is called a single-panel system and is described, for example, in "A new bright single panel LCD-projection system without mosaic color filter, Nakanishi et al., ITEJ Technical Report Vol. 19, No. 8, pp. 1-5 (1995). A single-panel system requires a resolution which is three times higher than that for a three-panel system.
When the LCD pixels in the light valve 2 are arranged in a matrix, images enlarged and formed on the screen 6 by the projection type display device will also be arranged in a matrix. If the LCD pixels on the light valve 2 are arranged in stripes, the images on the screen 6 would also be in stripes. If the lenticular lens 5 has a periodic structure in the horizontal direction, moire patterns would appear because of the interference in the spatial frequency between the period of pixel images in delta matrix or in the stripes and the periodic structure of the lenticular lens in the horizontal direction. Moire patterns render the images projected on the screen to be less legible.
Various techniques have been developed to prevent or suppress the effect of moire patterns. For example, Japanese Patent Application Laid-open No. 62-236282 discloses a display device wherein the pitch of pixels of the projected images as against the pitch of the lenticular lens (pitch ratio) is two-fold or less. Japanese Patent Application Laid-open No. 2-97991 discloses a display device wherein the pitch ratio is set so that the longest moire wavelength would be the minimum. More specifically according to the latter technique, the effect of moire patterns is mitigated by selecting the pitch ratio P/PL so that it satisfies n+1/2 (N; is a positive integer), given the pixel pitch P of the projected image and the pitch PL of the lenticular lens of the screen.
However, when a screen designed according to said prior art method is used for an LCD panel with a certain type of matrix arrangement, highly conspicuous moire patterns have been observed to appear on the screen.